Automotive emissions control is a mature industry. Automakers and suppliers have been challenged to control and reduce vehicle tailpipe emissions by the U.S. Clean Air Act in 1965 and subsequent legislation in other countries. Base engine emissions of controlled exhaust products have been reduced significantly over the past thirty years, as compliance with periodically decreasing tailpipe limits has been made possible through the use of catalytic converters.
A catalytic converter typically contains one or more catalytic elements, which lower hydrocarbons (HC), carbon monoxide (CO) and/or nitrogen oxides (NOx). Some catalyst systems are also designed to reduce particulate matter from diesel engines.
The individual catalysts may be coated onto ceramic or metal spheres or on metal screens which act as particulate filters. Most often, however, the active catalyst components are coated onto a ceramic or metal honeycomb element termed a “monolith.” Also included in this category are monoliths applied as diesel particulate filters (DPFs). The active catalyst components may be supplied either directly to the monolith, or more typically as a component of a “washcoat” which can be an aqueous slurry of particulate supports such as metal oxides that are impregnated with the active catalytic component. The applied washcoat can be calcined, or may be deposited on the monolith after the metal oxide-supported catalyst has been calcined. A single catalytic converter or multiple converters may be used. As many as four or five monoliths may be placed in succession in the exhaust stream depending on the particular application.
Emissions requirements have become increasingly stringent, requiring development of both new catalysts and higher catalyst loadings. In addition to absolute emissions standards, emissions control system longevity, i.e. “durability”, requirements have also been extended. This maintenance of operation requirement over extended periods has also challenged catalyst development, and has required still further increased catalyst levels. It is primarily the catalyst loading levels, in particular, precious metal loading, which controls the cost of the catalytic converter. Converters that meet all the requirements with a minimum of precious metal loading is a primary objective of catalyst manufacturers.
Numerous reactions can occur during combustion of a hydrocarbon fuel in a variety of temperature and fuel/air stoichiometric environments and the products of such reactions can limit catalyst durability. For example, it was recognized quite early that lead, formerly supplied as an octane booster in fuel as tetraethyl lead, was a serious catalyst poison. The lead octane boosters, thus, have been removed from modern day fuels.
Other octane boosters added to non-leaded fuel, such as methylcyclopentadienyl manganese tricarbonyl (MMT), may result in manganese contained in the exhaust gas and which in excess amount, may coat, foul, or otherwise poison the catalytic layers. Thus, numerous trace elements still come into contact with the automotive exhaust catalysts, some unavoidably so, and several of these are known to decrease catalyst durability. Not all these poisonous trace elements are derived from the fuel.
For example, zinc dialkyldithiophosphates (ZDDPs) have been long used as antioxidants and/or high-pressure lubricant additives in motor oils. Especially with modern high-speed engines, increased piston/wall clearances and decreased sealing allow increased entry of oil into the combustion chamber, where oil additives, or their combustion byproducts, subsequently pass into the exhaust stream. Such catalytic poisoning is one of the primary obstacles to the durability of low emission catalyst systems. Trace amounts of zinc, phosphorus, calcium, and other elements are put in engine oil as anti-wear additives. The purpose of such additives is to protect engine parts from excessive wear during start-up, when engine oil is not coating the metal components of the engine. However, as the engine burns oil, zinc and phosphorus are exhausted through the catalytic converter, which may accelerate degradation of exhaust catalyst activity. Although the antiwear additives could be removed from the oil, long-term durability of the engine could suffer.
The use of engine anti-wear additives, such as phosphorous and zinc, is described in many references. These additives include compounds such as ZDDPs, also referred to as zinc dithiophosphates (ZDTPs), and zinc dithiocarbamates (ZDTCs). Other disclosed zinc and phosphorous additives to oil include metallic detergents included as extreme pressure agents. Reference is made to U.S. Pat. Nos. 4,674,447 and 5,696,065. The phosphorous and zinc are disclosed as lowering the function of the motor vehicle exhaust treatment catalyst.
Automotive oil additives, such as ZDDP, form an antiwear coating on engine components and act as an antioxidant in the oil. Although engines are designed to minimize the quantity of engine oil exiting the engine via the combustion chamber and exhaust system, it is inevitable that a small fraction of engine oil is released by this mechanism. The ZDDP additive of engine oil deleteriously affects catalytic converters due to phosphorus from the ZDDP interfering with active sites within the catalyst. These phosphorus containing species deposit onto, or react with washcoat components, such as aluminum oxide and cerium oxide, and remain there indefinitely. This phenomenon is commonly referred to as “phosphorus poisoning.”
This poisoning mechanism is quite complex, and highly dependent upon the operating temperature, the oil consumption of the engine, and the source of the oil consumption. For example, when oil leaks past the piston rings, and washes into the combustion chamber, the oil goes through the combustion process. This will result in certain types of phosphorus and/or zinc compounds (among other contaminants). Particular compounds may have a particular deactivation effect on the catalytic converter, depending upon the operating condition. On the other hand, oil that leaks past the exhaust valve guide and stem, may not go through the combustion process, and result in a different type of poisoning of the catalytic converter, namely, forming a glaze layer covering the catalytic layers.
Measures to eliminate or reduce ZDDP in engine oils have been investigated. Alternatives to ZDDP have been produced which have been shown to provide antioxidant and antiwear properties similar to ZDDP. However, the ZDDP alternatives are cost prohibitive. Engine oils may be formulated with a lesser amount of ZDDP with the consequences that engine wear and oil oxidation increase, the former limiting engine life and the latter reducing useful oil life.
It is well known in the art to utilize catalyst compositions to treat gaseous streams such as the exhaust gases of internal combustion engines. It is also well known that sulfur oxides (SOx) and phosphorous oxides (POx) tend to poison, i.e., deactivate many catalysts used for such treatment. SOx is a particular problem inasmuch as it is generated by the oxidation of sulfur compound impurities often found in gasoline and diesel fuel. POx is often generated from phosphorous compounds in engine lubricating oils. It is known in the art to place a guard (e.g., alumina) or filter ahead of a catalyst to attempt to protect the catalyst from SOx and/or POx. However, difficulties are encountered when the guard or filter becomes saturated with SOx and/or POx.
It was proposed in Japanese applications JP 55 151109 and JP 56 044411, to insert an alumina-containing phosphorus trap in the oil recirculation system to remove suspect components from the oil being recirculated, and thus protect the exhaust catalyst. However, such systems are inefficient in the degree of protection achieved, may become rapidly fouled, and may remove desirable antioxidant from the oil.
It is known in the prior art to use combinations of sorbents and catalysts to reduce catalytic poisoning. This is shown, for example, at pages 45-48 of the publication Environmental Catalysis For A Better World And Life, Proceedings of the 1st World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italiana of Rome, Italy, in an article entitled “The New Concept 3-Way Catalyst For Automotive Lean-Burn Engine Storage and Reduction Catalyst”, by Takahashi et al. This article deals with NOx abatement in lean NOx gases and shows materials comprising precious metals, mainly platinum, and various alkaline and alkaline earth metal oxides, mainly barium oxide and rare earth metal oxides, disposed on supports such as alumina. At page 47 of the article, there is disclosed the concept of employing NOx storage compounds and catalytic components dispersed on a common support material.
U.S. Pat. No. 5,202,300, “Catalyst For Purification of Exhaust Gas”, issued on Apr. 13, 1993, to M. Funabiki et al, discloses a catalyst composition comprising a refractory support having deposited thereon an active layer containing a palladium and rhodium catalytic metal component dispersed on alumina, a cerium compound, a strontium compound and a zirconium compound.
U.S. Pat. Nos. 4,714,694, 4,727,052, and 4,708,946 disclose the use of bulk cerium oxide (ceria) to provide a refractory oxide support for platinum group metals other than rhodium. Highly dispersed, small crystallites of platinum on the ceria particles may be formed and stabilized by impregnation with a solution of an aluminum compound followed by calcination.
Japanese Patent Publication No. 52530/1984 discloses a catalyst having a first porous carrier layer composed of an inorganic substrate and a heat-resistant noble metal-type catalyst deposited on the surface of the substrate and a second heat-resistant non-porous granular carrier layer having deposited thereon a noble metal-type catalyst. The second carrier layer is formed on the surface of the first carrier layer and has resistance to the catalyst poison.
Japanese Patent Publication No. 31828/1985 discloses a catalyst for purifying exhaust gases comprising a honeycomb carrier and a noble metal having a catalytic action for purifying exhaust gases. The carrier is coated with two slurries containing different kinds of alumina powder. Subsequently, the alumina-coated honeycomb is immersed in a noble metal solution.
Japanese Patent J-63-205141-A discloses a layered automotive catalyst in which the bottom layer comprises platinum or platinum and rhodium dispersed on an alumina support containing rare earth oxides, and a topcoat, which comprises palladium and rhodium dispersed on a support comprising alumina, zirconia and rare earth oxides.
Japanese Patent J-63-077544-A discloses a layered automotive catalyst having a first layer comprising palladium dispersed on a support comprising alumina, lanthana and other rare earth oxides and a second coat comprising rhodium dispersed on a support comprising alumina, zirconia, lanthana and rare earth oxides.
U.S. Pat. No. 4,587,231 discloses a method of producing a monolithic three-way catalyst for the purification of exhaust gases. A mixed oxide coating is applied to a monolithic carrier by treating the carrier with a coating slip in which an active alumina powder containing cerium oxide is dispersed together with a ceria powder and then baking the treated carrier. Platinum, rhodium and/or palladium are then deposited on the oxide coating by a thermal decomposition. Optionally, a zirconia powder may be added to the coating slip.
U.S. Pat. No. 4,923,842 discloses a catalytic composition for treating exhaust gases comprising a first support having dispersed thereon at least one oxygen storage component and at least one noble metal component, and having dispersed immediately thereon an overlayer comprising lanthanum oxide and optionally a second support. The layer of catalyst is separate from the lanthanum oxide. The noble metal can include platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component can include the oxide of a metal from the group consisting of iron, nickel, cobalt and the rare earths. Illustrative of these are cerium, lanthanum, neodymium, praseodymium, etc.
Engine technology and exhaust gas treatment technology have reduced the level of lubricating oil, including phosphorous and zinc compounds, passed by engines to the exhaust treatment catalysts, and the catalysts have been sufficiently active to treat exhaust gases in accordance with various government regulations. However, as engine performance continues to increase and environmental regulations become more stringent, exhaust catalyst activity will have to be increased and maintained with longer engine life, for example, 150,000 miles. It is common also that the oil consumed by an engine increases as the mileage increases (e.g., >100,000 miles). Accordingly, there will be a greater build up of compounds, particularly phosphorous and/or zinc compounds and others, passing to the emission treatment catalyst from the engine. Low emission vehicles could benefit from exhaust after treatment systems with a tolerance for engine oil or fuel additive poisons.
It is desirable to have a poisoning resistant catalyst that maintains its functionality as both engine performance and lifespan increase. It would be desirable to provide a means whereby catalyst poisons, which lower emission catalyst durability, can be effectively removed or sufficiently tolerated without requiring increased precious metal catalyst loading to compensate for reduced catalyst activity.